The present invention relates to an optical attenuator including dual control loops, the attenuator being suitable for use in optical communication systems where control of optical power is required. Moreover, the invention also relates to a method of controlling the optical attenuator using two control loops.
Conventional optical communication systems comprise a plurality of nodes interconnected by optical fibre waveguides for conveying communication traffic between the nodes. In such systems, it is current practice to employ wavelength division multiplexing (WDM) where the optical radiation propagating along the waveguides comprises radiation components distributed into a number of wavebands having mutually different frequencies, each component conveying an associated portion of the communication traffic. Optical amplification is included at various nodes in the systems to maintain radiation power level, thereby improving signal-to-noise ratio and extending communication range possible. Such optical amplification is conventionally provided by optically-pumped erbium-doped optical fibre amplifiers (EDFA). EDFAs are inherently non-linear devices; hence, it is important that input radiation provided thereto is within a reasonable operating range. Insufficient input radiation power can result in threshold effects in the devices causing amplified radiation output therefrom to suffer erratic fluctuations. Conversely, excess input radiation can result in saturation effects within the devices because of finite laser pumping power available in the devices. Characteristics of the devices for both insufficient input radiation power and excess input radiation power can result in errors being introduced into data conveyed in radiation amplified by the devices; such introduction of errors is a serious problem in communication systems where system clients demand a high degree of communication reliability.
For addressing the aforementioned problem, it is conventional practice to employ a line build out (LBO) attenuator prior to each EDFA to ensure that input radiation supplied to the EDFA is within a range over which the EDFA functions without introducing errors into the radiation propagating therethrough. Moreover, it is conventional practice to monitor radiation power provided from the attenuator to the EDFA and compare it with a reference level corresponding to optimum power input to the EDFA; attenuation provided by the attenuator is then adjusted by a negative feedback circuit to maintain the radiation power input to the EDFA at nominally the reference level.
Such an arrangement comprising an LBO attenuator connected prior to an associated EDFA operates satisfactorily in practice for a relatively static situation where fluctuations in input optical power to the attenuator occur gradually so that the attenuator and its associated feedback circuit are able to track the fluctuations. However, in conventional communication systems, abrupt interruptions in optical power supplied can occur, for example when the systems are being reconfigured to incorporate new add-drop multiplexers.
When such an interruption occurs, the feedback circuit associated with the attenuator will respond to the interruption by reducing attenuation provided by the attenuator because the interruption is analogous to substantially a zero level of input radiation power. There subsequently arises a transitory problem when optical power is reapplied to the attenuator after the interruption; the attenuator will be set to a low attenuation by the feedback circuit during the interruption so that the EDFA on reapplication of the radiation to the attenuator will be overloaded until the feedback circuit has an opportunity to react by increasing the attenuation provided by the attenuator to an acceptable level as before prior to the interruption. Such overload will result in corruption of data conveyed in the radiation whilst readjustment in the feedback circuit occurs. The attenuator and its associated circuit exhibit a time response which is many orders of magnitude greater than the duration of data pulses conveyed in the radiation; hence, data corruption will continue to occur until the feedback circuit has reacted.
A conventional solution for coping with such an interruption in the input radiation is to hold the feedback circuit so that the control signal provided to the attenuator for controlling its attenuation is held during the duration of the interruption at its value immediately prior to interruption. Although, to a first order, such holding of the control circuit can reduce overload from occurring in the EDFA, the attenuator itself can be prone to drift with regard to attenuation provided even if the control signal applied thereto is held constant. Such drift can occur due to changes in one or more environmental factors affecting the attenuator.
The inventors have appreciated that the circuit can be modified to include a second negative feedback circuit linked to temperature of the attenuator to hold the temperature of the attenuator substantially constant during the interruption, the attenuator providing a characteristic that radiation propagating therethrough is subject to an attenuation dependent upon temperature of the attenuator.
According to a first aspect of the present invention, there is provided an optical attenuator for a communication system, the attenuator including:
a) attenuating means for receiving optical input radiation and for optically attenuating the input radiation to generate corresponding optical output radiation; and
b) controlling means for controlling attenuation provided by the attenuating means,
characterized in that
c) on application of the input radiation to the attenuating means, the controlling means is operable using a first feedback loop to monitor the output radiation and to regulate its power to a predetermined level by controlling a physical attenuation determining parameter of the attenuating means; and
d) on interruption of the input radiation to the attenuating means, the controlling means is operable using a second feedback loop to monitor the physical parameter and to regulate the parameter to a predetermined level.
The invention provides the advantage that the attenuator is capable of maintaining the attenuating means attenuation at a level which will not result in overload when the input radiation is reapplied after an interruption thereof.
The attenuator preferably includes optical amplifying means for receiving and amplifying the output radiation from the attenuating means to generate amplified output radiation for output from the attenuator. Inclusion of the amplifying means enables the attenuator to selectively provide both amplification and attenuation for the input radiation transmitted through to the amplified output radiation. Advantageously, the amplifying means includes an erbium-doped optical fibre amplifier. Such an amplifier is prone to overload and to saturation, but the attenuating means and controlling means are operable to counteract overload in the amplifier. Conveniently, the controlling means is operable to regulate the physical parameter on interruption of the input radiation so that overload is avoided within the attenuator on subsequent reapplication of the input radiation.
The predetermined level during the interruption is preferably a value of the parameter immediately prior to the controlling means switching from controlling the attenuating means using the first loop to controlling the attenuating means using the second loop. Alternatively, the predetermined level can be a fixed set value which is not modified.
The attenuator in practice exhibits a finite response bandwidth. In order to avoid transient settling disturbances of the controlling means when switching between the first loop and the second loop, it is preferable that the controlling means is operable to switch from the second loop to the first loop a settling period after reapplication of the input radiation after an interruption.
In order for the attenuator to switch at an appropriate instance to the second loop, it is desirable that the controlling means includes detecting means for detecting occurrence of an interruption of the input radiation, said interruption being determinable from abrupt changes in the radiation power of the output radiation, and for prompting the controlling means to switch to using the second feedback loop to control the attenuating means.
In some circumstances, the communication system can issue a warning signal that interruption is imminent. It is therefore desirable that the controlling means includes detecting means for detecting occurrence of interruption of the input radiation, said detecting means operable to interpret warning data supplied to the detecting means indicative of an imminent input radiation interruption.
Advantageously, attenuation provided by the attenuating means is determined by the temperature of attenuation determining optical parts thereof. Thus, conveniently, the aforementioned physical parameter is a temperature of at least part of the attenuating means. In other words, the attenuating means is preferably operable to provide optical attenuation of the input radiation to generate the output radiation dependent upon the temperature of the attenuating means.
For practical convenience of both heating and cooling the attenuating means to control its attenuation, the attenuating means preferably includes a thermoelectric element controllable from the controlling means for heating or cooling the attenuating means, thereby controlling optical attenuation within the attenuating means.
For achieving enhanced power efficiency in the attenuator, the controlling means is preferably operable to drive the thermoelectric element with a pulse width modulated (PWM) electrical drive signal for controlling power input to the thermoelectric element and thereby controlling the temperature of the attenuating means. Switched PWM control provides the benefit that power loss in driver electronic circuits generating the PWM drive signal is less compared to equivalent circuits operable in a non-switching mode.
Conveniently, the attenuating means includes a thermistor sensor for sensing the temperature of the attenuating means and for providing a measure of the temperature of the attenuating means to the controlling means for use when the second loop is activated. The thermistor, when included within a potential divider, is capable of providing a useful magnitude of temperature indicative signal for the controlling means. In contrast, potentials developed by thermocouple sensors are often in the order of microvolts and require considerable amplification before acquiring a suitable magnitude suitable for the controlling means.
Conveniently, the controlling means comprises one or more microprocessors, and the first and second feedback loops are implemented in software executable on said one or more microprocessors. Software implementation of the feedback loops provides flexibility, especially when said one or more microprocessors are required to exchange data or communicate with other microprocessors present in the communication system.
According to a second aspect of the present invention, there is provided a method of controlling an optical attenuator for a communication system, the attenuator including:
a) attenuating means for receiving optical input radiation and for optically attenuating the input radiation to generate corresponding optical output radiation; and
b) controlling means for controlling attenuation provided by the attenuating means,
characterized in that the method includes the steps of:
c) on application of the input radiation to the attenuating means, monitoring the output radiation and using a first feedback loop of the controlling means to regulate power of the output radiation to a predetermined level by controlling a physical attenuation determining parameter of the attenuating means; and
d) on interruption of the input radiation to the attenuating means, monitoring the physical parameter and using a second feedback loop of the controlling means to regulate the parameter to a predetermined level.